Optimal functioning of the nervous system requires selective wiring of neural circuits, the precision of which is achieved through experience-dependent refinement after birth. A classical model system of the experience-dependent development is the ocular dominance plasticity in the visual system, where monocular eyelid closure in a critical period of early life leads to a shift of cortical responses towards the non-deprived eye. Despite decades of work, it is still unknown what purpose the critical period serves during normal development, when the inputs from the two eyes are intact. In this grant, the proposed work aims to determine what cortical function is shaped by normal vision-induced plasticity during the critical period and to reveal its underlying molecular and synaptic mechanisms. First, the investigators will test whether the critical period plasticity drives the matching of binocular orientation preference during normal development. Both single unit recording and two-photon calcium imaging will be performed in the mouse visual cortex to determine the time course of binocular matching of orientation preference and its requirement of normal visual experience in the critical period. Second, with genetically or pharmacologically altered level of inhibition, which is known to shift the timing of the critical period of ocular dominance plasticity, the investigators will determine whether intracortical inhibition controls the timing of binocular matching by regulating the maturation of orientation selectivity. Intracellular whole cell recording will also be performed in vivo to reveal the spatiotemporal patterns of synaptic inhibition in mediating the binocular matching of orientation preference and in regulating the critical period timing. Finally, the receptive field structure of individual cortical neurons will be studied separately to the two eyes at different developmental stages to reveal how receptive fields change monocularly during the critical period to mediate binocular matching of orientation preference. Pharmacological experiments will also be carried out to determine if cortical activity and NMDA receptor activation are required for the binocular matching process. Together, these studies will reveal a physiological role for the critical period in normal development. Because ocular dominance plasticity and its critical period is a model system for human amblyopia and strabismus, a full understanding of cortical changes that normally take place during development will have important implications for the understanding and treatment of these diseases.